The ability to visualize biomolecules within living specimen by engineered fluorescence tags has become a major tool in modern biotechnology, cell biology, and life science. Encoding fusion proteins with comparatively large autofluorescent proteins is currently the most widely applied technique. As synthetic dyes typically offer better photophysical properties than autofluorescent proteins, alternative strategies have been developed based on genetically encoding unique tags such as Halo- and SNAP-tags, which offer high specificity but are still fairly large in size. Small tags like multi-histidine or multi-cysteine motifs may be used to recognize smaller fluorophores, but within the cellular environment they frequently suffer from specificity issues as their basic recognition element is built from native amino adds side chains. Such drawbacks may be overcome by utilizing bioorthogonal chemistry that relies on attaching unnatural moieties under mild physiological conditions.
Powerful chemistry that proceeds efficiently under physiological temperatures and in richly functionalized biological environments is the copper(I) catalyzed Huisgen type (3+2) cycloaddition between linear azides and alkynes, or the inverse electron-demand Diels-Alder (4+2) cycloaddition reaction between a strained dienophile such as trans-cyclooctene or norbornene and a 1,2,4,5-tetrazine, both forms of click chemistry (Kolb et al., Angew Chem Int Ed Engl 2001, 40:2004; Devaraj at al., Angew Chem Int Ed Engl 2009, 48:7013). However, the more established (3+2) cycloaddition requires a copper catalyst that is toxic or bacteria and mammalian cells, which strongly reduces biocompatibility of this type of click chemistry. This limitation has been overcome by Bertozzi and co-workers, who showed that the click reaction readily proceeds without the need for a cell-toxic catalyst when utilizing ring-strained alkynes as a substrate (Agard et al., J Am Chem Soc 2004, 126:15046). Copper-free click chemistry has found increasing applications in labeling biomolecules. Fluorescent dyes comprising cyclooctynyl groups were used to label carbohydrates and proteins comprising enzymatically attached azide moieties in vivo (Chang et al., Proc Natl Acad Sci USA 2010, 107:1821) and the labeling of alkyne-/cycloalkyne-modified phosphatidic acid with azido fluorophores is described in Neef and Schultz, Angew Chem Int Ed Engl 2009, 48:1498. The alternative. Diels-Alder (4+2) cycloaddition, for labeling molecules in vivo requires the reaction of a strained dienophilic group such as a trans-cyclooctenyl group or norbornenyl group with a 1,2,4,5-tetrazine fused to a small molecule probe, e.g. a fluorophore (Devaraj et al., Bioconjugate Chem 2008, 19:2297; Devaraj et al., Angew Chem Int Ed Engl 2009, 48:7013; Devaraj et al., Angew Chem Int Ed Engl 2010, 49:2869). No catalyst is required.
The translational modification of proteins by direct genetic encoding of fluorescent unnatural amino acids using an orthogonal aminoacyl tRNA/synthetase pair offers exquisite specificity, freedom of placement within the target protein and, if any, a minimal structural change. This approach was first successfully applied by Summerer et al. (Proc Natl Acad Sci USA 2006, 103:9785), who evolved a leucyl tRNA/synthetase pair from Escherichia coli to genetically encode the UAA dansylalanine into Saccharomyces cerevisiae. In response to the amber stop codon TAG, dansylalanine was readily incorporated by the host translational machinery. This approach has meanwhile been used to genetically encode several small dyes and other moieties of interest. For instance, engineered Methanococcus jannaschii tyrosyl tRNAtyr/synthetase, E. coli leucyl tRNAleu/synthetase as well as Methanosarcina maize and M. barkeri pyrrolysine tRNApyl/synthetase pairs have been used to genetically encode azide moieties in polypeptides (Chin et al., J Am Chem Soc 2002, 124:9026; Chin et al., Science 2003, 301:964; Nguyen et al, J Am Chem Soc 2009, 131:8720, Yanagisawa et al., Chem Biol 2008, 15:1187). However, due to the need to evolve new aminoacyl tRNA/synthetase pairs and potential size limitations imposed by the translational machinery, larger dyes with enhanced photophysical properties and other bulky moieties have not yet been encoded.
Despite large efforts, there is still a high demand for strategies to facilitate site-specific labeling of proteins in vitro and in vivo. Thus, it was an object of the present invention to provide amino acids or analogs thereof that can be translationally incorporated in polypeptide chains and allow labeling of the resulting polypeptide in vitro as well as in vivo.